As shown in FIG. 1, the photoresist flow passage provided in the photoresist applying device of prior art is structured by a photoresist bottle 1, a buffer tank 2, a pump 3, a filter 4, an ejection valve 5, a suck-back valve 6, nozzle pipe 7b, and a nozzle tip 8. The parts of from the photoresist bottle 1 to the ejection valve 5 are respectively connected by the pipe 7a. 
At first, in a waiting state of the conventional photoresist applying device, the photoresist stored in the photoresist bottle 1 is previously filled up to a tip end of the nozzle tip 8 through the various parts from the buffer tank 2 to the nozzle pipe 7. When applying photoresist to a semiconductor substrate 100 surface, the photoresist is ejected at the nozzle tip 8 by an ejecting operation of the pump 3 and the simultaneous opening/closing operation of the ejection valve 5. Furthermore, by the immediately after continuing sucking operations of the pump 3, photoresist is sucked from the photoresist bottle 1 into the pump 3, to prepare for the next ejection.
Incidentally, in order to prevent photoresist from dripping off the tip end of the nozzle tip 8 after ejecting photoresist, the suck-back valve 6 is in a mechanism to suck photoresist back from the nozzle tip 8 side, which is usually in an integral structure with the ejection valve 5 in many cases. The buffer tank 2 has an operation that, in the case an air bubble is mixed in a supply pipe 7a interior during exchanging the empty photoresist bottle 1, the air bubble is removed from a drain pipe attached on the buffer tank 2 by pressurizing the photoresist bottle 1 with using an N2 gas or the like, and a function to detect a fact the photoresist bottle 1 is emptied of photoresist by a medical-fluid sensor provided on the buffer tank 2.
However, in the photoresist applying device of the above structure, there is a conspicuous problem that air bubbles occur at various points of the photoresist flow passage of from the photoresist bottle 1 to the nozzle tip 8 and stay in the photoresist within the pipe 7a. Although this can be considered responsible for gas/liquid separation in the photoresist liquid, the major factor is assumably by the influence of negative pressure as caused by sucking operation of the pump 3 or sucking operation of the diaphragms of the ejection valve 5 and suck-back valve 6, or natural separation within the filter 4.
In this manner, the air bubbles caused or staying at various points of the photoresist flow passage move in the photoresist pipe 7a, to be ejected together with photoresist from the nozzle tip 8 to a semiconductor substrate 100 (hereinafter substrate) surface depending upon photoresist ejecting operation of the pump 3. The air bubble ejected on the substrate 100 surface, during spin-coating photoresist to the substrate 100 surface, flows and moves radially over the substrate 100 surface from a center toward the outer periphery of the substrate 100 surface. Finally, it is spun out of the substrates 100 surface into vanishing by rotation of the substrate 100. At that time, with respect to the even film thickness of the photoresist film entirety applied over the substrate 100 surface, the film thickness varies over a path the air bubble have flowed, to appear as radial color unevenness in external appearance. In the uneven color region, because of film thickness variation, integrated circuits are caused partial variation in wiring dimensions during the subsequent integrated-circuit printing process onto the substrate 100 surface, resulting in a cause of trouble such as circuit performance instability or poor operation when made into a complete product.